1. Technical Field of the Invention
The present invention relates generally to an electromagnetic isolation apparatus for electronic equipment such as a portable telephone, a cordless telephone, or a selective calling receiver which is designed to prevent undesired electromagnetic waves or noise from entering or leaking out of the electronic equipment.
2. Background Art
FIG. 5 shows a radio terminal unit having a conventional isolation structure which includes a shield case 1, a control circuit substrate 2, a radio circuit substrate 4 having mounted thereon radio communication circuit components 3 such as a transmitter and a receiver, a printed grounding conductor 5, and grounding terminals 6.
The control circuit substrate 2 has mounted thereon control circuit components and is attached to an upper wall of the shielding case 1. The shield case 1 is made of a molded resin case coated or plated with metal. The radio circuit substrate 4 is attached to the bottom of the shield case 1. This type of isolation structure is disclosed, for example, in Japanese Patent First Publication No. 4-215500.
The printed grounding conductor 5 is formed on the radio circuit substrate 4 and is connected to the shield case 1 through the grounding terminals 6. Each of the grounding terminals 6 has an elastically deformable conductive strip 7 engaging an inner wall of the shield case 1. Such a structure is disclosed, for example, in Japanese Utility Model First Publication No. 62-160599.
The above isolation structure, however, has the following drawback. The substrates 2 and 4 are both thin, and the shield case 1 is made from resin material so that they are subject to set or deformation. The attachment of the deformed substrate 2 or 4 to the shield case 1 will develop clearances therebetween through which undesired electromagnetic waves enter or leak out. In order to avoid this drawback, the prior art just made it necessary to manufacture the substrates 2 and 4 and the shield case 1 with high accuracy, but such an approach results in an increase in manufacturing cost.
The elastically deformable conductive strips 7 of the grounding terminals 6 disposed between the radio circuit substrate 4 and the shield case 1 urge the substrate 4 to move away from the shield case 1, thereby increasing the clearance between the substrate 4 and the shield case 1. This causes more undesired electromagnetic waves to enter or leak out through the clearance, leading to malfunctions of, for example, the electronic components 3 or external circuits placed near the radio terminal unit.
The above isolation structure also uses two printed circuit boards; the control circuit substrate 2 and the radio circuit substrate 4, for improving the isolation performance. This type of printed circuit board is usually passed through a reflowing heater to mount thereon a plurality of electronic components. Different shapes of printed circuit boards encounter the difficulty in passing through the same heater, thus requiring use of printed circuit boards of the same size or different reflowing heaters one for each printed circuit board. This results in an increase in design limitation or investment in plants and equipment.